This invention generally relates to toolholders, and is specifically concerned with an endmill adapter of the type having an integral collet that has been improved by the provision of a torque reducing locknut and collet interface.
An endmill is a generally cylindrical cutting tool which has a plurality of cutting edges around its outer periphery. The endmill usually includes a cylindrical shank which fits into an axial bore of a toolholder. For precision cutting operations, the endmill shank must be securely and concentrically mounted within the toolholder bore. To this end, various mechanisms have been developed for securing the endmill and the toolholder against axial movement and against rotation of the endmill with respect to the holder. One prior art securing mechanism employs a set screw threadedly engaged in the wall of the holder which is tightened against a flat formed on the shank of the endmill. Unfortunately, the radial set screw creates an imbalance in the holder which creates unwanted vibration and eccentric movements of the tool during use, particularly at high rotational speeds. Still another prior art securing mechanism has been the combination of a collet and chuck. A tapered collet having an axial bore for receiving the shank of the endmill is radially compressed by a locknut that is threadedly connected to the body of the toolholder. In theory, the collet collapses uniformly around its circumference so that the shank of the tool is accurately positioned at the center of the holder. A non-pullout wedge may be used in combination with the collet to prevent axial and rotational movement of the shank of the endmill relative to the interior of the collet. A collet and chuck arrangement of this type is disclosed in U.S. Pat. No. 4,341,006 to Starn. However, the interfacing surfaces of the collet and chuck body require a substantial amount of accurate machining which translates into substantial manufacturing costs. Further, the cumulative tolerances between parts limit the degree of accuracy which can be obtained.
To overcome the shortcomings associated with the use of prior art set screw and collet and chuck mechanisms, engineers employed by Kennametal Inc. developed an endmill adapter having an integral collet described and claimed in U.S. Pat. No. 5,234,296 by Presby and Mogilnicki. The collapsible segments of the collet disclosed in the '296 patent are formed by cutting longitudinal slots into an integrally formed, frustro-conical skirt extending down from the tapered shank of the toolholder. The longitudinal slots do not extend all the way to the ends of the collet segments, which has the effect of forming a continuous tie ring that integrally joins the distal ends of all of the collet segments. The base of the resulting collet is then threaded for receiving the threaded interior of the locknut. The resulting collet is easy and inexpensive to manufacture, and the continuous tie ring that interconnects the distal ends of the collet segments provides greater rigidity at the outer end of the collet for improved accuracy. In use, a generally tubular locknut having a threaded inner diameter is screwed over the threads at the base of the collet for deflecting the collet segments radially inward to grip the shank of an endmill or tool.
While the endmill adapter disclosed and claimed in the '296 patent constitutes a substantial advance in the art, the inventors have noted several areas where this collet might be improved. For example, while the greater rigidity of the collet segments defined by the longitudinal cuts in the frustro-conical collet body does indeed improve the accuracy with which this device grips and centers the shank of an endmill, the resulting rigidity of the collet segments also increases the amount of torque necessary to turn the locknut to adequately radially deflect the segments into firm engagement with a tool shank. Additionally, as is the case with other prior art collet chuck mechanisms, it may be possible for errant sprays of coolant to enter the chuck mechanism and flow between the tool shank and the interior surface of the collet. The presence of liquid coolant between the interior surface of the collet and the tool shank secured therein is undesirable, as it can interfere with the desired frictional grip between these two components.
Clearly, what is needed is an improved endmill adapter of the type that utilizes an integral collet in which the locknut can be turned in order to radially compress the collet segments with only low or moderate amounts of torque to facilitate a quick and easy change of tools in the collet. Ideally, such an improved collet would further have a means for preventing errant flows of coolant from entering the space between the tool shank and the inner surface of the collet.